In general, switched capacitor sampling filters are made using a so-called "ladder" structure whose basic elements are lossless discrete integrators (LDIs) for lowpass filters or lossless discrete differentiators (LDDs) for highpass filters.
These basic elements comprise capacitors, switches, and a single amplifier. The order of the sampling filter is equal to the number of amplifiers, i.e. it is equal to the number of lossless discrete integrators or differentiators. A switched capacitor sampling filter using LDIs has the advantage of being insensitive to stray capacitance and of being better adapted to integrated circuits than a filter using LDDs. Unfortunately, a filter using LDIs is not satisfactorily in the frequency band of highpass filters since it suffers from problems of instability due to the use of a signal flowgraph for calculating the values of the capacitor elements in the filter, which approach does not take account of termination effects specific to highpass filters. In other words, the flowgraph of LDIs is well adapted to designing lowpass filters but does not apply properly to designing highpass filters.
In order to solve this problem, the article "SC MODIFIED LOSSLESS DISCRETE DIFFERENTIATORS AND RESULTING SC HIGHPASS LADDER FILTERS", Electronic Letters, Jan. 16, 1986, pages 97-99, proposes a method using a modified lossless discrete differentiator (MLDD) in order to obtain a switched capacitor sampling filter of the highpass type. The basic cell is an LDD which is modified and organized as a highpass filter. The use of an LDD makes it possible to obtain a sampling filter which is very stable and insensitive since its flowgraph applies well to the filter frequency band of highpass filters. Never-the-less, the drawback of this type of circuit is that the so-called "z" linear transform of the cell used to pass from the continuous frequency domain to the sampled frequency domain includes a second order term, i.e., a third order sampled filter would require six operational amplifiers, for example.
Such a circuit is therefore not satisfactory since it includes too high a number of operators, thereby increasing its manufacturing cost. In addition, the performance of the filter is degraded since it includes a capacitance which is highly sensitive to stray capacitance since it is bipolar switched. Finally, the calculation of the values of the filter elements from a predetermined filter characteristic when synthesizing a filter to meet predetermined requirements is approximate since the calculation is applicable, when designing highpass filters, only at high sampling frequencies, and this requires the use of differential amplifiers having a high gain-passband product.
A method is known in digital filtering for inverting the polarity of the LDD-type z-transform of a basic cell in order to obtain a highpass type digital filter. This method makes use of LDI-type lowpass filter design and changes various signs in the basic digital filter circuit. These sign changes are easily performed, for example by changing the sign of an adder. Unfortunately, this method used in digital filtering is difficult to apply to sampled filtering since it gives rise to technological problems which are difficult to solve. In particular, heretofore, changing the sign of the z-transform of a sampling filter requires technologically complicated operations using numerous amplifiers, capacitors, and switches. This increases the cost of manufacturing such filters in the form of integrated circuits.
The object of the present invention is to provide a solution to these problems. To this end, the invention provides a circuit suitable for transposing the filter frequency band of a switched capacitor sampling filter, e.g. to transform a lowpass type switched capacitor sampling filter into a highpass type switched capacitor sampling filter.